Bubbles user education for stack & manage

Two types of education
1) highlighting the bubble when it first appears on screen (after user tap)
2) highlighting manage button when a bubble is expanded

* Both of these are added in BubbleStackView & respond to theme changes
* Prefs to save whether user has seen either education
* Simple animations in / out; we can tune them later
* Adjusts the starting position of the stack to work with user education
  positioning
* Adds a secure setting string to force user education to show for demo
  purposes.

Test: manual:

1) get bubble for first time
2) tap on flyout => flyout disappears & user education shows
3) get another update to that bubble => note that it doesn't show bc
   user education is displaying

1) get bubble for first time
2) tap on bubble => flyout disappears & user education shows
3) tap outside of the user education => user education hides

1) get bubble for first time
2) wait => after flyout disappears, user education shows
3) tap  on the bubble => user education hides & bubble expands, after
   expansion "manage" user education appears
4a) interact with content in the activity view
4b) collapse the stack (via back or tapping outside)
4c) tap on the manage button
4d) select a different bubble
=> each of 4x) should dismiss the manage button user education

Check that switching themes (and dark mode) gets the right colour for user
education backgrounds & text.

Bug: 148105621
Change-Id: I8e6401573c1ac21dfe636aa1132461fe04ce67ac
Merged-In: I8e6401573c1ac21dfe636aa1132461fe04ce67ac